Blood flow to the body's organs and tissues is regulated to meet their metabolic demands. O2-dependent control of arteriolar tone has been implicated in this regulatory process for over a century. However, the cellular site where changes in PO2 are sensed, and how changes in PO2 are coupled to changes in arteriolar muscle tone remain unclear. Therefore, the overall aims of the research proposed are to 1.) identify the cellular location of the sensorthat mediates arteriolar O2 reactivity, and 2.) establish the mechanism of action of O2 on these microvessels. The focus of this proposal will be on arteriolar O2 reactivity, in the hamster cheek pouch, a common microcirculatory model. We are in a unique position to study arteriolar function in this tissue from the level of single ion channels in isolated arteriolar muscle cells in vitro, to responses of intact arterioles in the living microcirculation. The proposed studies will: acquire new information on the cellular location where changes in PO2 are sensed; establish the ionic basis of O2-induced changes in arteriolar tone; and improve our understanding of the mechanism of action of cysteinyl-leukotrienes (cysLTs) on arterioles in the microcirculation. These data will not only aid in understanding the basic physiology of blood flow regulation in the microcirculation, but may also provide new clues to the etiology or consequences of diseases that impact the microcirculation such as hypertension, diabetes, and atherosclerosis and well as diseases such as asthma, inflammatory bowel disease, psoriasis, renal disease, and eosinophilic inflammation in which cysLTs are implicated. The specific aimsof the research are to test five hypotheses concerning the site and mechanism of action of O2 on hamster cheek pouch arterioles. They are: 1. Cells that synthesize cysLTs (the signaling molecules that we propose mediated arteriolar O2 reactivity in the cheek pouch) are distributed, extravascularly along arterioles, 2. O2 and cysLTs depolarize, elevate [Ca2+], and contract arteriolar muscle cells by stimulating Ca2+ influx through L-type Ca2+ channels, which subsequently activate Ca2+-dependent Cl-efflux through Ca2+-activated C1 about channels, 3. O2- and cysLT-induced depolarization and constriction are limited by activation of Ca2+activated K+ channels, 4. cysLTs initiate vasoconstriction that can be conducted along arterioles through an arteriolar muscle cell pathway and 5. 02-induced constriction is conducted along arterioles through an arteriolar muscle cell pathway. These hypotheses will be tested by integration of state-of-the-art methodologies including immunohistochemistry; laser scanning confocal microscopy; intravital videomicroscopy, ratiometric calcium measurements, enzymatic isolation of arteriolar muscle cells; single-channel and whole-cell patch clamp recording; and conventional microelectrode recording in cheek pouch arterioles from golden Syrian hamsters.